Phospholipase C-gamma1 (PLC-gamma1) contains two tandem SH domains. Upon stimulation of cells with platelet-derived growth factor (PDGF), PLC-gamma1 binds to the tyrosine-phosphorylated PDGF receptor through one or both of its SH2 domains, is phosphorylated by the receptor kinase, and is thereby activated to hydrolyze phosphatidylinositol 4,5- bisphosphate. The individual roles of the two SH2 domains of PLC-gamma1 in interaction of the enzyme with the PDGF receptor have now been investigated by functionally disabling each domain. A critical Arg residue of each SH2 domain was mutated to Ala, the wild-type and mutant PLC-gamma1 proteins were transiently expressed in a PLC-gamma1- deficient fibroblast cell line, and the effects of PDGF stimulation of the transfected cells were investigated. The mutant protein in which the COOH-terminal SH2 domain was disabled bound to the PDGF receptor, was phosphorylated by the receptor, and catalyzed the production of inositol phosphates to extents similar to those observed with the wild- type enzyme. In contrast, the mutant in which the NH2-terminal SH2 domain was impaired did not bind to the PDGF receptor and consequently was neither phosphorylated nor activated. These results suggest that the NH2-terminal SH2 domain, but not the COOH-terminal SH2 domain, of PLC-gamma1 is required for PDGF-induced activation of PLC-gamma1. - Phospholipase C-gamma, platelet-derived growth factor, Src homlogy 2 domain, protein tyrosine kinase, Phosphoinositide